Heat-softenable toners are widely used in imaging methods such as electrostatography, where electrically charged toner is deposited imagewise on a dielectric or photoconductive element bearing an electrostatic latent image. Generally in such methods, the toner is then transferred to a surface of another substrate, such as a receiver sheet of paper or a transparent film, where it is then fixed in place to yield the desired final toner image. Heat and pressure, in combination are commonly utilized to fix or fuse the toner to the receiver. The heat and pressure are often applied by a pair of opposed members, such as a pair of rollers. As the toner bearing receiver passes between through the nip between the rollers; one of them, usually referred to as a "fuser roll" is heated and contacts the toner-bearing surface of the receiver sheet. The other roller, usually referred to as a pressure roll, presses the receiver sheet against the fuser roll.
The fuser roll usually comprises a rigid core covered with a resilient material, which will be referred to herein as a "base cushion layer." The resilient base cushion layer and the amount of pressure exerted by the pressure roll serve to establish the area of contact of the fuser roll with the toner-bearing surface of the receiver sheet as it passes through the nip of the pair of rolls. The size of this area of contact helps to establish the length of time that any given portion of the toner image will be in contact with and heated by the fuser roll. The degree of hardness (often referred to as "storage modulus") and stability thereof, of the base cushion layer are important factors in establishing and maintaining the desired area of contact.
Pressure and fuser rolls can have a regular cylindrical shape; however, an advantage is provided in some applications if the rolls are shaped to provide a gradient in pressure along a direction parallel to the axes of the rolls. This can be accomplished by, for example, continuously varying the overall diameter of one of the rolls along the direction of its axis such that the diameter is smallest at the midpoint of the axis and largest at the ends of the axis, in order to give the roll a "bow tie" or "hourglass" shape. The resulting pair of rolls will exert more pressure on the receiver sheet near the ends of the rolls than in the middle. Since a heated roll is generally more subject to permanent deformation on use than is an unheated roll, hourglass shaped, unheated pressure rolls are commonly used with cylindrical, heated fuser rolls. This provides a longer useful life for the more complexly shaped component, but does not address the problem of deformation of the fuser roll. As it is used, the fuser roll permanently deforms to the shape of the pressure roll. This degrades and eventually eliminates the pressure gradient.
In the past, it had been thought that various materials' suitability for use in fuser roll base cushion layers in terms of their stability during use--i.e., their ability to resist degradation (as evidenced by weight loss), creep (permanent deformation), and changes in hardness, during use in fuser rolls--could be determined by subjecting samples of the materials to conditions of continuous high temperature and continuous high stress (i.e., pressure), and then measuring the resultant changes in weight, shape (e.g., length), and hardness (e.g., storage modulus). This has since been disproven. Static testing is not a very good predictor of the stability that materials will exhibit during actual use in fuser roll base cushion layers. It has been found that testing based upon the application of cyclic stress is a better predictor of behavior of materials during actual use.
Fuser roll materials can be conveniently tested under conditions of cylic stress using a Mechanical Energy Resolver (also referred to herein as an "MER"). This device applies heat continuously to maintain the samples at a constant elevated temperature. The device also applies stress to the samples in the form of a compressive force, but does so in a manner such that the amount of compressive force applied varies cyclicly (i.e., sinusoidally). The results of such testing consistently correlate with, and therefore reliably predict, the degree of stability a material will exhibit in the base cushion layer of a fuser roll during actual use.
Another consideration for fuser rolls is the materials that will contact the rolls during use. In a typical electrophotographic process fusing subsystem there are multiple sets of rollers. In order to prevent toner build-up on the rollers, image degradation, hot offset, and toner contamination problems which may decrease fuser roller life, release oil is often applied to the fusing roller. The release oil is typically poly(dimethyl)siloxane oil (also referred to herein as "PDMS oil"), which is selected for its ability to withstand the almost continuous high temperatures (.about.200.degree. C.) of the electrophotographic fusing process. While PDMS oil does an excellent job in its role as release agent, its compatibility with PDMS-based roller materials results in swelling of the rollers. This swelling cannot be easily compensated for, since it is generally non-uniform. Paper passing over the rollers can wick away some of the release oil within the paper path, resulting in a differential availability of the release oil to roller areas within and outside the paper path. This causes differential swell of the roller inside and outside the paper path so that a "step pattern" is formed in the roll. This can cause problems when different size papers are used and can lead to increased wear and decreased roller life.
One type of material that has been widely employed in the past to form a resilient base cushion layer for fuser rolls is condensation-crosslinked poly(dimethylsiloxane) (also referred to herein as "PDMS") elastomer. The prior art has also taught or suggested that various fillers comprising inorganic particulate materials can be included in such PDMS base cushion layers to improve their mechanical strength and/or thermal conductivity. Higher thermal conductivity is advantageous when the fuser roll is heated by an internal heater, so that the heat can be efficiently and quickly transmitted toward the outer surface of the fuser roll and toward the toner on the receiver sheet it is intended to contact and fuse. Higher thermal conductivity is not so important when the roll is intended to be heated by an external heat source. Disclosure of such filled condensation-cured PDMS elastomers for fuser rolls can be found, for example, in U.S. Pat. Nos. 4,373,239; 4,430,406; and 4,518,655.
One specific example of a condensation-crosslinked PDMS elastomer, which contains about 32-37 volume percent aluminum oxide filler and about 2-6 volume percent iron oxide filler, and which has been widely used and taught to be useful in fuser rolls, is sold under the trade name, EC4952, by the Emerson Cummings Co., U.S.A. However, it has been found that fuser rolls containing EC4952 cushion layers exhibit serious stability problems over time of use, i.e., significant degradation, creep, and changes in hardness, that greatly reduce their useful life. MER test results correlate with and predict the instability exhibited during actual use. Nevertheless, materials such as EC4952 initially provide very suitable resilience, hardness, and thermal conductivity for fuser roll cushion layers.
Some condensation-crosslinked PDMS elastomers that show less change in hardness and creep than EC4952 or aluminum oxide-filled PDMS are disclosed in U.S. patent application Ser. No. 08/167,584 (tin oxide filler), U.S. Pat. Nos. 5,292,606 (zinc oxide filler), 5,269,740 (copper oxide filler), 5,292,562 (chromium oxide filler), and 5,336,539 (nickel oxide filler).
U.S. patent application Ser. No. 08/306,066 discloses a tin oxide-filled, addition cured polysiloxane system containing 0 to &lt;20 mol% diphenyl units and the remainder dimethyl units. U.S. patent application Ser. No. 08/268,136 teaches a zinc oxide-filled, condensation cured polydimethyl diphenyl siloxane system containing 20-40 wt% zinc oxide and &lt;20 mol% polydiphenylsiloxane.
U.S. Pat. No. 4,970,098 by J. Ayala-Esquilin, W. H. Dickstein, J. L. Hedrick, Jr., J. C. Scott, and A. C. Yang discloses a diphenyl-dimethylsiloxane elastomer filled with 40-55 wt% zinc oxide of 100-500 nm particle size, 5-10 wt% graphite of &lt;10 .mu.m particle size, and 1-5 wt% ceric dioxide of 0.2-3 .mu.m particle size. The diphenyl content was 20-50 wt% (equivalent to 8.5,to 27 mol%).
U.S. Pat. No. 4,807,341 by P. A. Nielsen and J. A. Pavlisko discloses a diphenyl-dimethylsiloxane elastomer containing 5-15 mol% diphenylsiloxane and 0-5% vinyl-addition crosslinked siloxane units. Aluminum oxide and iron oxide fillers were disclosed.
U.S. Pat. No. 4,074,001 describes fixing rollers for electrophotography which may comprise phenyl-substituted diorganopolysiloxanes filled with calcium carbonate (&lt;10 .mu.m particle size), iron oxide (&lt;10 .mu.m particle size), and titanium dioxide (&lt;10 .mu.m particle size).
U.S. Pat. No. 4,360,566 describes heat fixing rollers for electrophotography that may comprise addition-crosslinked diphenyl-substituted polyorganosiloxanes, filled with substantial amounts (50-250 parts by weight) of siliceous filler.
U.S. Pat. No. 4,454,262 describes silicone rubbers that may contain phenyl radicals and that contain spindle-shaped calcium carbonate filler.
The above references have a variety of shortcomings. Most do not address the issue of improved stability under cyclic stress at elevated temperature optionally accompanied by reduced oil swell. Some of the references call for fillers that a costly.
It would therefore be very desirable to be able to provide a fuser roll with a layer comprising a addition-crosslinked PDMS elastomer containing zinc-oxide filler, wherein the layer material exhibits good stability under conditions of elevated temperature and cyclic stress, i.e., good resistance to degradative weight loss, creep, and changes in hardness, and optionally good resistance to PDMS oil.